Self-calibrating multiple field of view telescope for remote atmospheric electromagnetic probing and data acquisition

ABSTRACT

The invention is a transmitting and receiving telescope for use in generalized electromagnetic radiation communication systems, including atmospheric probing systems. The telescope optics, electromagnetic radiation source laser and receiver are coaxially aligned along the telscope axis. The telescope can be constructed with one received field of view or with a plurality of received fields of view. The telescope mirrors have apertures along the telescope axis to allow alignment laser pulse or CW radiation to travel along the telescope axis without being reflected. The preferred electromagnetic radiation source is a laser which can operate in both TEM00 and TEM01 modes. The TEM00 mode is employed for alignment purposes since the energy of this mode is concentrated along the axis. The TEM01 mode is used for data acquisition because the energy of this mode is concentrated in a donut shaped region having its hole centered on the axis. Constant intensity illumination is produced in the viewed area during data acquisition by separating the donut into inner and outer annuli along the line of maximum intensity and imaging the two beams to provide 100 percent overlay at the range of interest. A preferred use for this telescope is in atmospheric probing LIDAR systems for measurements of the motion and concentration of the atmospheric environment, particularly pollution measurements. This LIDAR system uses a real time data processing system employing 800 megabit analog to digital converters and a correlation system to transform the acquired data into useable form in real time.

ited States Patent i Kadrmas [76] Inventor: Kenneth A. Kadrmas, 7909Westhaven Dr., Huntsville, Ala. 35802 [22] Filed: Aug. 2, 1972 [21]Appl. N0.: 277,437

[52] U.S. Cl 250/206, 350/55, 350/294,

250/216, 356/4 [51] Int. Cl. G0ln 21/26 [58] Field of Search 250/216,218, 209,

[56] References Cited UNITED STATES PATENTS 3,411,852 11/1968 Marinozzi,Jr. 356/211 3,455,623 I 7/1969 Harris 350/55 3,510,225 5/1970 Collis356/4 3,113,989 12/1963 Gray et a1. 250/209 Primary ExaminerWalterStolwein Attorney-L. D. Wofford, Jr. et a1.

[ 5 7 ABSTRACT The invention is a transmitting and receiving telescopefor use in generalized electromagnetic radiation communication systems,including atmospheric probing systems. The telescope optics,electromagnetic radiation source laser and receiver are coaxiallyaligned along the telscope axis. The telescope can be constructed withone received field of view or with a plurality of received fields ofview. The telescope mirrors have apertures along the telescope axis toallow alignment laser pulse or CW radiation to travel along thetelescope axis without being reflected. The preferred electromagneticradiation source is a laser which can operate in both TEM and TEM modes.The TEM mode is employed for alignment purposes since the energy of thismode is concentrated along the axis. The TEM mode is used for dataacquisition because the energy of this mode is concentrated in a donutshaped region having its hole centered on the axis. Constant intensityillumination is produced in the viewed area during data acquisition byseparating the donut into inner and outer annuli along the line ofmaximum intensity and imaging the two beams to provide 100 percentoverlay at the range of interest.

A preferred use for this telescope is in atmospheric probing LIDARsystems for measurements of the 13 Claims, 13 Drawing Figures SER 22LASER RELAY OPTICS 30 ORIGIN OF THE INVENTION The invention describedherein was made by an employee of the United States Government and maybe manufactured and used by or for the government for governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates to the field of transmittinglreceiving telescopes and to theiruse in LIDAR systems.

2. Prior Art The prior art contains many telescopes which are designedfor use as combination transmitting and receiving telescopes. Thesetelescopes employ on axis mirrors to reflect either the transmitted beamor the received beam, or both, from or to, off axis systems. Suchtelescopes are difficult of alignment and are quite sensitive tovibrations which can cause misalignment. These telescopes are alsounduly bulky because of the housing extensions which are necessary tohouse the off axis systems, i.e., laser sources and receivers.

Prior art laser probing systems which are also known as LIDAR systems(for Light Detection and Ranging) employ a single telescope which makescalibration of the returned signal difficult and prevents the use ofcorrelation techniques in obtaining meaningful reduction of the data toinformation which is'readily interpreted.

OBJECTS A primary object of this invention is to provide an easilyaligned transmitting and receiving telescope.

Another object is production of a single telescope having a plurality ofreceived fields of view.

Another object is to provide a telescope having no partially reflectingon axis mirrors.

A further object is to produce a telescope having a laser source,transmission optics, receiver optics and a receiver sensing system, allof which are in coaxial alignment aling the telescope axis.

A still further object is toprovide a transmitting and receivingtelescope which directly'measures the power of each transmitted laseroutput'pulse for calibration of the receiver s sensors to establish anabsolute standard for determining the percentage of the radiation whichis reflected back to thetelescope.

Another object is provision of a telescope which provides constantintensity illumination across the viewed area to eliminate problems ofdetermining whether a change in received signal is due to a change inatmospheric reflectivity or a change in beam position.

Still another object of the invention is to provide a fan beam telescopewhich is easily aligned.

Still another object of the invention'is to provide a coaxial telescopehaving a common focalpoint for the transmitted and received beams.

Still another object of the invention is to provide an improved LIDARsystem for measuring atmospheric pollutants.

A further object is the provision of a LIDAR pollution measuring systemwhich measures multiple points with each laser output pulse to providecomparison data for prompt drift rate measurements.

A still further object is to provide a LIDAR telescope having multiplefields of view.

A still further object is to provide a LIDAR system which producesreduced and correlated data in realtime.

SUMMARY The above and other objects and advantages are achieved byprovision of a multiple mirror telescope having a common, on axis, focalpoint for both the transmitted and received light beams. The transmittedrays make a smaller angle with the axis than the received rays. Thelarger angle formed by the received rays and the telescope axis isemployed to separate the received beam from the transmitted beam. Theelectromagnetic radiation source which is preferably a laser is placedon axis and focused at the focal point by any appropriate optics.Sensors for measuring the intensity of the received beam are placedalong the inside of a ring having the telescope axis as its axis.

Substantially uniform illumination of the area being probed is obtainedby separating the donut shaped transmitted beam along its line ofmaximum intensity and focusing the two beams thus obtained to overlap atthe area being probed.

Multiple received fields of view are obtained by dividing the returnedbeam into a plurality of beams and focusing each beam on a differentphotosensor.

The use of coaxial geometry makes the laser source easily changed whenit is desired to change the frequency of the probing electromagneticradiation.

The telescope system is made self-calibrating by the use of a commonfocal point for both the transmitted and received beams and theinclusion of a luminescent gas within the telescope enclosure. At thefocal point the transmitted laser pulse is so intense that the gasexhibits luminescence. This luminescence is received by a photosensorthrough the receiver optics. The inten- 'sity of the luminescence is adirect measure of the transmitted pulse power, provided the laser outputpulse is circularly polarized.

The pollution measuring LIDAR system employs two of the multiple fieldsof view transmitting and receiving telescopes. The telescopes are spacedapart and aimed at a common measurement volume in the sky. Theproportion of the laser light which is reflected back to the telescopereceiving sensors is measured as a function of time to determine thereflecting cross section of the measurement volume as a function ofdistance from the telescope. The use of two telescopes producesincreased measurement accuracy and pollutant distribution analysis.

A computer data acquisition system employs 800 megabit analog to digitalconverters and correlation techniques to reduce the quantity of datawhich must be stored, while simultaneously producing the data inrealtime, in a form which is readily analyzed to determine significantcharacteristics of the atmosphere and pollutant distribution therein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is the preferred embodiment ofthe telescope. FIG. 2 is an alternate embodiment of the telescope.

FIGS. 3a and 3b show the characteristics of the output pulse produced bythe laser when it is operating in the TEM,,,* mode.

FIGS. 4a and 4b show the characteristics of the output pulse produced bythe laser when it is operating in the TEM. mode.

FIG. 5 shows the secondary mirror which is used to focus the receivedradiation on the photosensors of the receiver.

FIG. 6 shows a parabolic mirror, sections of which are employed in themirror of FIG. 5.

FIG. 7 shows the LIDAR system and the measurement volume.

FIGS. 8a and 8b show possible photosensor outputs for two differentfields of view for the same laser output pulse.

FIG. 9 shows the correlation of the photosensor outputs in FIGS. 8a and8b.

FIG. 10 is a block diagram of the electronic data acquisition system.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of thetelescope is shown in cross section in FIG. 1. To prevent confusion, theoptical system is shown without mechanical supports because theinvention is in the optics and the sup-. ports are conventional.

Telescope 20 comprises four separate systems, a laser source 22, laserrelay optics 30, main telescope optics 40 and receiver 60.

For the most efficient use of the system, laser 22 must be able toproduce an output pulse in both TEM. and TEM modes. The electromagneticradiation produced may be either visible or invisible in accordance withthe intended use of the telescope.

The characteristics of a TEM laser output mode are shown in FIG. 4. FIG.4a is the illumination pattern produced by TEM mode, point 152 being thepoint of maximum intensity and circle 154 being at the half power point.FIG. 4b shows the power distribution as a function of the distance fromthe center of the beam. TEM radiation is produced by most lasers.

FIG. 3 shows the characteristics of a TEM.,,* laser output mode. FIG. 3ais the illumination pattern produced by the TEM mode, circle 142 beingthe line of maximum intensity and circles 144 and 146 being the halfpower points. A laser exhibiting this output mode is sometimes referredto as a donut laser because the beam illuminates a circular regioncontaining a central non-illuminated area. FIG. 3b is a diagram of theintensity of a TEM f mode as a function of distance from the axis.

Whichever mode the laser is operating in, the laser output beam consistsessentially of parallel rays of electromagnetic radiation. The use of alaser facilitates the easy changing of the frequency of the probingelectromagnetic radiation. A slight axial displacement of the new laserfrom the position of the old laser will produce no effect on thefocusing of the beam, since the parallel rays assure that the opticalsystem will produce the same transformation of the parallel raysindependent of small axial displacements of the laser.

The uses to which the two different laser output modes are put will bediscussed below in connection with the alignment and operation of thetelescope.

In the diagram of FIG. 1, the transmitted and received rays overlap andcross in the main telescope optics 40. To maintain the clarity of FIG.1, within the main optics portion of the telescope, the transmitted raysare traced below the axis and the received rays are traced above theaxis although it will be understood that both the transmitted andreceived beams are rotationally symmetric about the telescope axis.

Since the transmitted and received rays do not overlap or cross in thelaser or receiver relay optics portions of the telescope both sets ofrays are traced above and below the axis in these portions of FIG. 1.

Laser relay optics 30 focus a donut laser beam to a point at focal point25 with the proper angle of convergence for the beam to strike a firstsecondary mirror 42 in the main telescope optics 40. Laser relay optics30 comprise a secondary mirror 34 and a primary mirror 38. These mirrorsreflect the donut laser beam causing it to converge to a point at focalpoint 25. Those skilled in the art will understand that although thistwo mirror system is the preferred mechanism for focusing the laser atfocal point 25, other systems including lenses may be used.

Both of the mirrors of laser relay optics 30 have apertures along thetelescope axis. These apertures allow a TEM laser beam to travel downthe telescope axis without being reflected or absorbed. This allowsrapid alignment of the telescope as will be explained in detailhereinafter.

Main telescope optics 40 comprise two primary mirrors and two secondarymirrors, all of which are rotationally symmetric and are coaxiallyaligned along the telescope axis. The first secondary mirror 42 islocated furthest from focal point 25, first primary mirror 46 is nextfurthest from focal point 25, while second secondary mirror 56 isbetween focal point 25 and first primary mirror 46. The second primarymirror 50 is located between the second secondary mirror 56 and focalpoint 25. Mirror 50 may preferably have two separate portions, atransmitting portion 48 and a receiving portion 52. Each of the mirrorshas an aperture along the telescope axis to allow TEM radiation totravel the length of the telescope without being reflected or absorbed.

A donut laser output pulse after being focused to a point at focal point25 by laser relay optics 30 diverges and strikes secondary mirror 42.Mirror 42 reflects the transmitted rays'back toward primary mirrors 46and 48. First primary mirror 46 intercepts an inner annulus 408 of thedonut pulse, while an outer annulus 410 passes the outer edge of firstprimary mirror 46 and strikes the transmitting portion 48 of the secondprimary mirror 50. This separation of the transmitted donut pulse intotwo different beams can be best understood by referring to FIG. 1wherein the intensity pattern of the donut pulse is superimposed on thebeams ray tracings. When the telescope is properly aligned, the line ofmaximum intensity 142 of the donut pulse will occur at the edge ofmirror 46 whereby the inner portion 408 of the pulse, is reflected bymirror 46 and the outer portion 410 of the pulse, will pass mirror 46and be reflected by mirror 48. Both portions 408 and 410 of the donutpulse range from zero intensity to maximum intensity. Mirrors 46 and 48reflect the respective portions of the donut beam toward the area to beviewed. The two beams are aimed to fully overlap at the preferredviewing distance of less than 10 kilometers. As can be seen from FIG. 1,the transmitted laser output intensity in the viewed area issubstantially uniform (within about plus or minus percent) because thesplitting of the transmitted beam results in the high intensity portionof beam 408 overlapping the low intensity portion of beam 410 and thelow intensity portion of beam 408 overlapping the high intensity portionof beam 410. The intensities of the two beams thus add to produce asubstantially uniform illumination of the viewed or target area.

The angle at which the transmitted beams 408 and 410 are reflected bymirrors 46 and 48 determine the size of the illuminated target area 77.The reflection angles for mirrors 46 and 48 are determined by thegrinding of the mirrors. Therefore, the size of the target area cannotbe changed once the telescope has been assembled.

The receiving portion 52 of the second primary mirror 50 receivesradiation reflected from the target (viewed) area and reflects it ontothe secondary mirror 56 which focuses the returned radiation to a pointat focal point 25 (the samepoint at which the transmitted beam isfocused).

Receiver relay optics 60 receive the returned beam and focus it onphotosensors 72 which determine the intensity of the returned beam. Thisis accomplished by a primary receiver mirror 64 intercepting thereturned beam as it expands after passing through focal point 25. Mirror64 reflects the returned radiation back toward a secondary receivermirror 68 which focuses the re turned radiation on photosensors 72. Adiaphragm 73 is placed between secondary mirror 68 and each photosensor72 to control the percentage (image size) of the returned radiationwhich is allowed to impinge on the photosensor. Secondary mirror 68determines how many received fields of view the telescope has. If thismirror is ground as a single surface which reflects all incidentradiation to a common focal point, then the telescope will have a singlefield of view. If mirror 68 comprises a plurality of separate mirrorsections each of which focuses the radiation it receives onto adifferent photosensor, then the telescope will have as many fields ofview as there are such mirror sections. The preferred secondary mirrorprovides four different focal points so that the telescope has fourfields of view. The photosensors 72 are supported on a ring 70. Ring 70and secondary mirror 68 are preferably made rotatable together throughan angle of plus or minus 40. This rotation allows the position of thefields of view to be changed relative to the ground plane without thenecessity of rotating the entire telescope. A total rotation of 90 wouldcause the new field of view of one photosensor to be identical to theold field of view of an adjacent photosensor. A rotation of only plus orminus 40 for a total rotation of 80 is employed because the remaining 10are taken up by the spiders which support the mirrors in the maintelescope optics 40. The spiders would prevent received radiation fromreaching a photosensor positioned in the omitted 10.

A further understanding of the construction of secondary mirror 68 maybe obtained by referring to FIGS. 5 and 6. FIG. 5 is a perspective viewof mirror 68 and photosensor support ring 70, looking from the generaldirection of receiver primary mirror 64. The secondary mirror shown inFIG. 5 is one which produces the preferred four fields of view. As canbe seen from the diagram, mirror 68 focuses the returned radiation onfour different photosensors 72. To accomplish this mirror 68 is made ofsections of a parabolic mirror. One section of a parabolic mirror whichmay be employed is shown in FIG. 6. FIG. 6 shows one quarter of aparabolic mirror I60, a section of which is used as one of the foursections of mirror 68. Points A, B, C, D, E and F delineate theintersections of line segments which form the circumference of mirror68. These same letters are shown in FIG. 5 to provide an orientation asto the position of the section of parabolic mirror I60. The use of fourparabolic sections in this secondary mirror 68 provides four separatefocal points for the received beam and thus provides telescope 20 withfour received fields of view.

Telescope 20 is contained within a hermetically sealed housing in orderto prevent contamination of the mirrors by dirt and moisture. Thehermetic housing is charged with a quantity of an luminescent gas. Thespecific luminescent gas and its pressure are selected so that thetransmitted laser output pulse causes the gas to exhibit luminescence ina restricted region at focal point 25. The quantity luminescence thusexhibited may be measured to determine the intensity of the transmittedpulse.

Filters may be placed over the end of the telescope housing to preventbackground radiation from reaching photosensors 72. This filtering willimprove the signal to noise ratio at the output of sensors which respondto the background radiation.

An alternative embodiment of the telescope of this invention is shown inFIG. 2 as telescope 80. Telescope is in all respects similar totelescope 20 except that it employs only two mirrors in the maintelescope optics and does not provide for splitting the transmitted beamto provide substantially constant illumination in the viewed area.

A LllDAR system employing the multiple fields of view (MFOV) telescopeof this invention is shown in FIG. 7. Two MFOV telescopes 208 and 226are employed. The telescopes are preferably placed less than 5kilometers apart and aimed at the same measurement volume 250 of thesky. Both telescopes transmit the data they receive to an electronicdata acquisition system 430. This transmission may be by telephone wire,radar or laser beam.

The LIDAR data acquisition system 430 is shown in FIG. 10 and comprisesa data reception section 632 and a data reduction section 470. The datareception section is comprised of a separate channel for each telescopedata photosensor. Each of these channels comprises a low impedanceamplifier 436 to amplify the photosensor output, a 800 megabit analog todigital converter (ADC) $88 which is controlled by clock it, and a setof shift registers will 58 whose inputs are connected to the output ofthe ADC 138 in succession by a clocked rotating gate M2. The shiftregister outputs are connected to a high speed memory 462, in successionby a second clocked rotating gate 660.

Data reception section 432 receives the data from the photosensors inanalog form and converts it to equivalent sampled digital informationfor subsequent processing. Each transmitted laser output pulse causesthe data reception section to process one amplitude time (space) historyfor each channel. An amplitude time history is a record of the amplitudeof the photosensor output as a function of time.

High speed memory 462 provides temporary storage for the amplitude timehistory data and provides this data to data reduction section 470 whichintegrates and correlates the data to convert the data to a usefulformat. The data reduction system comprises an 8 bit per word N wordrotating register 474, for each photosensor 72. Where N is the number ofsampled data points stored in high speed memory 462 for each amplitudetime history and is a around 1000 with 1024 being preferred since it isa power of two. Both the photosensors 72 and rotating registers 474 willbe referred to with suffixes A, B, C, or D, when it is desirable todistinguish one channel from another, while no letter designation willbe used when there is no need to distinguish between the differentchannels. Hereinafter, the A channel will be arbitrarily designated as areference channel.

The output from rotating register 474A is connected to the input of a 8bit per word N word shift register 476A. The output of shift register476A is connected back to the input of rotating register 474A.

The output of each of the rotating registers 4743, C and D is connectedto one input of a corresponding multiplier 482B, C and D respectively.The output of each multiplier is connected to the input of acorresponding adder 484 whose output is connected to a correspondingsealer 486. The output of each sealer is connected to a correspondingstorage register 488 whose output is transmitted to a mass storagesystem 490.

The output of rotating register 474A is connected as a second input toeach muliplier 482 B, C, and D.

The operation of each rotating register is controlled by an initiating,timing and shifting control system 480. Timing system 480 is controlledby a background process controller which also controls mass storage 490.

The multipliers 482 B, C and D and their associate circuitry calculatethe correlation functions A vs. B, A vs. C and A vs. D respectively.Shift register 476A allows the time delay between the A amplitude timehistory and the other time histories to be controlled so that thecorrelation phase which yields the maximum peak correlation value may befound for each channel B, C and D.

Assuming that measurement volume 250 is 1.5 kilometers from thetelescope and that a minimum of 1000 equally spaced data points aredesired per amplitude time history, the speed requirements for ADC 428can be readily calculated.

At a speed of 3 X 10 meters/second, it takes electromagnetic radiation1.0 X 10 seconds to travel 3 kilometers (round trip distance). 1024 datapoint values must be obtained during this 1.0 X 10 second "time periodwhich requires that one data point be obtained each I X 10 seconds. Thisrequires that data point values be obtained at a rate of l X 10 datapoints per second. Since an amplitude resolution of 8 bits per datapoint is desired in order to obtain accurate data, this data point raterequires a ADC bit rate of 8 X 10 bits per second or 800 megabits persecond.

Returning to telescope 20, the following is the procedure for aligningthe telescope. First, the telescope is mechanically aimed at a targetarea which is preferably spaced from the telescope by the desiredviewing distance of less than 10 kilometers.

Laser 22 is then operated in a continuous output TEM mode. The laseroutput thus produced passes down the axis of the telescope unobstructed,emerges from the telescope and strikes the target area. The point ofmaximum illumination on the target area is marked. The laser is thenswitched to transmit TEM output mode which is reflected and focused bythe telescope optics. One of the two split portions of the donut beammay be blocked at the telescope output, since at that point the beams donot overlap. It is preferable to block the inner transmitted beam 408 sothat the first secondary mirror and the second primary mirror may bealigned.

These mirrors are aligned when the transmitted donut pulse 410 ispositioned so that the center of the donut is at the previously markedmaximum intensity point for the continuous TEM alignment beam. Thelocation of this donut pulse is then marked for future reference. Nowthe outer transmitted donut beam is blocked and the inner donut beam 408is allowed to strike the target. Mirror 46 is aligned so that theposition of the donut pulse coincides with that of the previously markeddonut pulse. Thereafter, intensity measurements are made to assure thatthe donut beam is being split at its line of maximum intensity. Thetelescope transmission optics are now aligned.

Next, second secondary mirror 56 is aligned to focus the viewed area atfocal point 25. Thereafter, receiver relay mirrors 64 and 68 are alignedto focus thereceived beam on the photosensors. The telescope is nowcompletely aligned and is ready for data acquisition operation.

OPERATION OF THE PREFERRED EMBODIMENT To obtain data, laser 22 isoperated in a pulsed TEM mode. The laser omits a donut output pulse ofelectromagnetic radiation which is reflected by secondary laser relayoptics mirror 34 onto primary laser relay optics mirror 36 which focusesthe laser output pulse at focal point 25. The radiation intensity atfocal point 25 causes the specially chosen gas within the telescopehousing to exhibit luminescence. Most of the luminescence is radiated ina backward direction, i.e., toward receiver primary mirror 64. That partof the luminescence which strikes mirror 64 in the receiver relay opticsis reflected to mirror 68 which in turn reflects the radiation towardsthe photosensors 72. The luminescence striking the photosensors producesan electronic pulse at the output of the photosensors which isproportional to the energy of the laser output pulse and providescalibration information for the received signals. If the pulse ofluminescence is so strong as to overload the photosensors and preventthe proper response to the received signal from the viewing area,

then the photosensors may be maintained inactive during the luminescentperiod. If the receiver photosensors are held inactive during theluminescent period, then a separate (fifth) sensor is provided whosesole purpose is to respond to the luminescence to provide an outputpulse for use in calibrating the receiver photosensors.

After the output pulse from laser 22 passes focal point 25, the diameterof the donut expands with distance from the focal point 25. The pulsethen strikes the first secondary mirror 42 of the main telescope optics40. Mirror 42 reflects the pulse toward first and second primary mirrors46 and 48. As was explained above, inner portion 408 of the pulse isreflected by mirror 46 and outer portion 410 of the pulse is reflectedby mirror 48. The reflected pulses then travel outward from the end ofthe telescope housing and overlap at the desired viewing range.

As the electromagnetic radiation travels outward toward the viewingarea, it is reflected and scattered by particles in the atmosphere. Someportion of the reflected radiation will be reflected toward thetelescope, thus as soon as the laser beams leave the telescope, thetelescope will begin to receive returned radiation. However, thereturned radiation which is of primary interest is that which isreturned after the end of the period of time required for the laseroutput pulse to travel to the near side of the measurement volume andback. The returned radiation strikes second primary mirror 52 and isreflected to the second secondary mirror 56 which reflects the radiationand focuses it at focal point 25. After passing through focal point 25,the returned beam expands and strikes receiver mirror 64 which reflectsit toward receiver mirror 68. Receiver mirror 68 reflects the receivedradiation to the photosensors 72. Photosensors 72 produce a continuouselectrical output which is proportional to the radiation being received.Diaphragms 73 are preset to control the quan tity (image size) ofradiation striking the photosensors to prevent them from becomingoverloaded when a large percentage of the transmitted radiation isreflected back toward the telescope.

In the preferred embodiment where mirror 68 comprises four separateparabolic sections which reflect the return beam toward four separatephotosensors, returned radiation is received only from four restrictedportions of the viewing area. When diaphragms 73 are set to restrict thequantity of radiation received, they also reduce the size of theindividual viewing areas, since radiation reflected from the outerportions of what would otherwise be one of the individual viewing areasis prevented from reaching the photosensor. The four separate viewingareas allow data on four separate areas to be received on each laseroutput pulse. These four viewing areas will produce essentiallyidentical data when sky conditions are homogeneous. However, where skyconditions are non-homogeneous, as in the area of a smokelplume wherethere is a gradient in the pollutant density, each individual viewingarea will produce significant independent data.

The telescope of this invention is designed for use in a LIDARatmospheric probing system such as is shown in FIG. 7. The measurementvolume 250 is preferably located less than kilometers from eachtelescope. Each telescope projects its transmitted beam through themeasurement volume 250. The viewing area 77 as shown in FIGS. 1 and 7has an inner circumference 78 and an outer circumference 79. Within eachviewing area four individual viewing areas are defined as a result ofthe configuration of mirror 68 which has been selected as the secondarymirror of the receiver relay optics. For telescope 201) these individualviewing areas are 212, 214, 216 and 218. Each of these viewing areas islocated within the transmitted beam of telescope 200. The individualviewing areas for telescope 220 are shown as 232, 234, 236 and 238;.Ifdesired, by rotating the receivers secondary mirror 68 andphotosensors 72, the beams may be aligned so that two viewing areas foreach telescope are in a common plane. Thus, viewing areas 214 and 218for telescope 200 and viewing areas 232 and 236 for telescope 220 areshown as being in a common plane. This results in radiation travelingfrom viewing area 214 passing through two volumes of the atmosphere (252and 254) through which radiation traveling from viewing areas 232 and236 also passes, respectively. Thus, any pollutant within the volume 252will be reflected in the data signal from the sensor for viewing area214 and viewing area 232. Similarly, any pollutant in volume 254 will bereflected in the signals from both viewing area 214 and 236. Viewingarea 218 intersects both viewing areas 232 and 236 in a manner similarto that of viewing area 214. These intersections comprise common volumes256 and 258. Four intersections of this variety can always be obtainedby rotating' the beams with respect to each other. Two additional commonvolumes may be obtained when the measurement volume is equidistant fromboth telescopes, so that their transmitted beams and individual viewedareas have both diverged a common amount. In that situation, the viewingareas which are not in the common plane will intersect. Thus, viewingarea 212 from telescope 2110 will intersect with viewing area 238 fromtelescope 220 while viewing area 216 of telescope 200 will intersectwith viewing area 234 of telescope 220, thus, creating common volumes260 and 262 respectively. Various other patterns of common volumes maybe obtained by proper orientation of the received fields of view. Thesecommon volumes give the system its outstanding data acquisitionabilities. However, the system produces superior data even when theindividual viewing areas of the two telescopes are not aligned togenerate common volumes.

Each telescope is intended to receive only radiation which is reflectedfrom its own transmitted pulse, since it is desired to have a definitedistance calibration to the amplitude time history.

The information which is received during data' acquisition will be morereadily understood by reference to FIGS. 8a, 8b and 9. In FIG. 8a anamplitude time history 300 for the output of the photosensor whichresponds to radiation received from one viewing area (.A) is shown. Line302 is the zero time point of this time history and is the point ofmaximum amplitude output of the calibration signal. The calibrationsignal can be taken to last from point 302 to point 304. The point ofmaximum amplitude of the calibration signal is also used to initiate thedata acquisition cycle, thus assuring proper timing. Between points 304and 306 is the period when signals are being received only from the areabetween the telescope and the measurement volume. This time period hasbeen greatly reduced in both FIGS. 3a and 8b because it produces norelevant data. The portion of curve 3110 which occurs between points 306and 303 can be referred to as the target time and is the photosensoroutput for radiation which is reflected by the measurement volume.

Curve 310 of FIG. 3b is an amplitude time history for the output of aphotosensor which responds to radiation received from a different viewedarea (B) on the same transmitted pulse. These amplitude histories arefor some pollutant volume such as a smoke plume from a factory. Curve3211 of FIG. 9 is a cross correlation signal for the amplitude timehistories shown in F168. 8a and 8b. Point 322 of P16. 9 marks thebeginning of the first measurement volume response at line 306 of FIG.30. Line 324 marks the maximum amplitude of the correlation function.The distance between lines 322 and 324 reflects the difference in thelength of time for the transmitted pulse to travel to and from the smokeplume in the two different viewing areas. This distance thereforereflects the difference in telescope to pollutant distance for the twodifferent viewing areas. It will be understood that the correlationfunction between each pair of amplitude time histories is calculateddigitally by data acquisition system 430.

During data acquisition, the amplitude time histories for all of theviewed areas are processed simultaneously by data reception section 432.Since the processing of is identical for each channel within section 432only one channel will be discussed. Duplicate equipment processes eachother channel similarly.

As the transmitted pulse passes through focal point 25, the gas in thevicinity of the focal point exhibits luminescence. The luminescencecauses photosensor 72 to produce a calibration pulse at its output. Thispulse initiates a data acquisition cycle.

Amplifier 436 amplifies the calibration pulse and applies it to theinput of ADC 438. Slave clock 440 causes ADC 438 to take a sample andproduce a digital output every x seconds, where x is such that 1024 datasamples will be taken during the desired length of the data cycle. Thus,if the target area is 1.5 kilometers from the telescope, a sample istaken once every l X 10- seconds. Similarly if the target area is 7.5kilometers from the telescope, one data sample is taken every X seconds.

After passing through focal point 25 the donut pulse expands and strikesfirst secondary mirror 42 and is reflected back toward the primarymirrors. Mirrors 46 and 48 reflect portions 408 and 410 toward theviewed area 77.

Once the transmitted pulses enter the field of view of photosensor 72,the photosensor will begin to receive radiation which is reflected fromparticles in the atmosphere. Every x seconds ADC 438 takes a sample ofthe sensor output until the full 1024 data samples have been obtained. 7

As each data sample is provided at the output of ADC 438, it is gated toa different one of shift-registers 444-458 for instantaneous storage.The contents of the shift registers are transferred to high speed memory462 in sequence, but interleaved with the outputs of the shift registersfrom the other channels.

Once all of the data values for the transmitted pulse are stored inmemory 462, the data is transmitted to data reduction section 470. Thedata from each channel is transferred to the corresponding rotatingregister 474A, B, C or D. Once the data is in the rotating registers,the background process controller takes control and data receptionsection 430 may start another data cycle.

The registers 474 are rotated in unison so that their outputs aresupplied to multipliers 482 simultaneously. Multipliers 482 produce attheir outputs, the product of their two inputs. These values are added,scaled and stored in registers 488 and then transferred to storage'.Upon completion of the rotation cycle, correlation functions A vs. B, Avs. C, and A vs. D are stored.

Once the rotating registers have completed a cycle, shift register 476is employed to reload register 474A with a delayed or advanced versionof the channel A amplitude time history. The correlation functioncalculation is then repeated. Once all of the correlation functions havebeen calculated with the relevant phases, the highest peak value foreach correlation function is retained for permanent storage as is itsphase. The rest of the correlation results are discarded. Thecorrelation functions B vs. C, B vs. D and C vs. D are preferablycalculated simultaneously on duplicate equipment.

The value of the calibration signal for each channel and the integratedvalue of each amplitude time history is also stored.

Thus, while 4096 data points were originally obtained per telescope foreach light pulse, only 20 data values are stored, four calibrationsignal values A, B, C and D; four integrated amplitude time historyvalues A, B, C and D; six peak correlation function values A vs. B, Avs. C, A vs. D, B vs. C, B vs. D and C vs. D; and finally the phasevalues for the six correlation functions. This is a data reductionpf 290times. A data reduction of 250 times may be obtained by normalizingevery value by the calibration signal values and not storing them.

The above is a full and complete description of the construction andoperation of the preferred embodiment of this invention. While theinvention has been described in terms of the preferred embodimentthereof, it will be understood by those skilled in the art that manyvariations can be made therein, both in the telescope system and in theLIDAR system without departing from the scope of the invention asdefined in the claims.

I claim:

1. A transmitting and receiving telescope system comprising:

electromagnetic radiation source means for producing a donut outputbeam;

transmission optics means for separating the donut output beam intoseparate beams directed toward a common target area, and;

receiver optics means for focusing radiation received from the targetarea onto photosensor means, said photosensor means producing anelectrical output which is a function of the instantaneous intensity ofthe radiation striking the photosensor means.

2. The apparatus of claim 1 wherein the receiver optics means separatesthe received radiation into a plurality of received beams and whereinthe photosensor means comprises a separate photosensor for each receivedbeam, whereby the telescope has a plurality of received fields of view.

3. The apparatus of claim 1 wherein the transmission optics meanscomprises:

first and second primary mirrors and a first secondary mirror; saidfirst secondary mirror intercepting and reflecting the donut output beamtoward the primary mirrors; said first primary mirror intercepting aninner annulus of the donut beam and reflecting it toward the targetarea; said second primary mirror intercepting an outer annulus of thedonut beam and reflecting it toward the target area. 4. The apparatus ofclaim 3 wherein the first and second primary mirrors reflect therespective portions of the donut beam so that they overlap at the targetarea whereby substantially uniform illumination of the target area isproduced.

5. The apparatus of claim 3 wherein the inner annular beam ranges inintensity from a minimum value to the maximum intensity of the originalbeam and the outer annular beam ranges in intensity from a minimum valueto the maximum intensity of the original beam.

6. The apparatus of claim 4 wherein each mirror has an aperture centeredon the telescope axis whereby radiation may traverse the telescope alongits axis without obstruction.

7. The apparatus of claim 3 wherein a portion of the second primarymirror receives radiation from the target area and reflects it onto asecond secondary mirror;

said second secondary mirror reflecting the received radiation onto aprimary receiver mirror in the receiver optics means;

said primary receiver mirror reflecting the received radiation onto asecondary receiver mirror which reflects and focuses the radiation ontothe photosensor means.

8. The apparatus of claim 7 wherein the receiver secondary mirror ismulti-sectional and focuses different portions of the received beam ontodifferent individual photosensors of the photosensor means, whereby'thetelescope has multiple received fields of view within the target area.

9. The apparatus of claim 7 wherein both the transmitted and receivedbeams are focused to a point at a common focal point, said common focalpoint being within a gas tight housing charged with sufficientluminescent gas for the transmitted beam to cause the gas to exhibitluminescence in the vacinity of the common focal point, whereby theresponse of the photosensor means to the luminescence provides a measureof the power of the transmitted beam and may initiate a data receptioncycle.

10. The apparatus of claim 7 wherein each mirror has an aperturecentered on the telescope axis to allow radiation to traverse thetelescope along its axis without obstruction.

11. A transmitting and receiving telescope comprising: electromagneticradiation source means for producing a donut output beam, said radiationsource means being coaxial with the telescope;

transmission mirror optics means for receiving the output pulse anddirecting it toward a target area;

receiver mirror optics means for receiving radiation from the targetarea and focusing it on photosensor means, said photosensor meansproducing an electrical output signal which is a function of theinstantaneous intensity of the radiation striking the photosensor means;

each mirror having an aperture along the telescope axis wherebyradiation may traverse the telescope along its axis without obstruction.

12. The apparatus of claim 11 wherein both the transmitted donut beamand the received beam are focused to a point at a common focal point andthe common focal point is within a gas tight housing which is chargedwith sufficient luminescent gas whereby the transmitted beam causes thegas to exhibit luminescence and the photosensor means is struck by theluminescence and produces an output signal which is a function of thetransmitted pulse intensity.

13. A LIDAR atmospheric probe system comprising: a pulse transmissionelectromagnetic radiation source for illuminating a target area;

a plurality of photosensors each responsive to radiation received from adifferent viewed area within the target area, each photosensor producingan electrical output signal which is a function of the instantaneousintensity of the radiation striking it;

data reception means comprising a separate channel connected to theoutput of each photosensor;

each channel comprising analog to digital converter means for samplingthe photosensor output signal and producing a digital representation ofthe output signal amplitude;

each channel being connected to memory means for storing the digitalvalues and; data reduction means connected to the memory means forreceiving the digital values, said data reduction means includingcorrelation means for obtaining the phase and peak correlation valuesbetween the responses of the dif-

1. A transmitting and receiving telescope system comprising:electromagnetic radiation source means for producing a donut outputbeam; transmission optics means for separating the donut output beaminto separate beams directed toward a common target area, and; receiveroptics means for focusing radiation received from the target area ontophotosensor means, said photosensor means producing an electrical outputwhich is a function of the instantaneous intensity of the radiationstriking the photosensor means.
 2. The apparatus of claim 1 wherein thereceiver optics means separates the received radiation into a pluralityof received beams and wherein the photosensor means comprises a separatephotosensor for each received beam, whereby the telescope has aplurality of received fields of view.
 3. The apparatus of claim 1wherein the transmission optics means comprises: first and secondprimary mirrors and a first secondary mirror; said first secondarymirror intercepting and reflecting the donut output beam toward theprimary mirrors; said first primary mirror intercepting an inner annulusof the donut beam and reflecting it toward the target area; said secondprimary mirror intercepting an outer annulus of the donut beam andreflecting it toward the target area.
 4. The apparatus of claim 3wherein the first and second primary mirrors reflect the respectiveportions of the donut beam so that they overlap at the target areawhereby substantially uniform illumination of the target area isproduced.
 5. The apparatus of claim 3 wherein the inner annular beamranges in intensity from a minimum value to the maximum intensity of theoriginal beam and the outer annular beam ranges in intensity from aminimum value to the maximum intensity of the original beam.
 6. Theapparatus of claim 4 wherein each mirror has an aperture centered on thetelescope axis whereby radiation may traverse the telescope along itsaxis without obstruction.
 7. The apparatus of claim 3 wherein a portionof the second primary mirror receives radiation from the target area andreflects it onto a second secondary mirror; said second secondary mirrorreflecting the received radiation onto a primary receiver mirror in thereceiver optics means; said primary receiver mirror reflecting thereceived radiation onto a secondary receiver mirror which reflects andfocuses the radiation onto the photosensor means.
 8. The apparatus ofclaim 7 wherein the receiver secondary mirror is multi-sectional andfocuses different portions of the received beam onto differentindividual photosensors of the photosensor means, whereby the telescopehas multiple received fields of view within the target area.
 9. Theapparatus of claim 7 wherein both the transmitted and received beaMs arefocused to a point at a common focal point, said common focal pointbeing within a gas tight housing charged with sufficient luminescent gasfor the transmitted beam to cause the gas to exhibit luminescence in thevacinity of the common focal point, whereby the response of thephotosensor means to the luminescence provides a measure of the power ofthe transmitted beam and may initiate a data reception cycle.
 10. Theapparatus of claim 7 wherein each mirror has an aperture centered on thetelescope axis to allow radiation to traverse the telescope along itsaxis without obstruction.
 11. A transmitting and receiving telescopecomprising: electromagnetic radiation source means for producing a donutoutput beam, said radiation source means being coaxial with thetelescope; transmission mirror optics means for receiving the outputpulse and directing it toward a target area; receiver mirror opticsmeans for receiving radiation from the target area and focusing it onphotosensor means, said photosensor means producing an electrical outputsignal which is a function of the instantaneous intensity of theradiation striking the photosensor means; each mirror having an aperturealong the telescope axis whereby radiation may traverse the telescopealong its axis without obstruction.
 12. The apparatus of claim 11wherein both the transmitted donut beam and the received beam arefocused to a point at a common focal point and the common focal point iswithin a gas tight housing which is charged with sufficient luminescentgas whereby the transmitted beam causes the gas to exhibit luminescenceand the photosensor means is struck by the luminescence and produces anoutput signal which is a function of the transmitted pulse intensity.13. A LIDAR atmospheric probe system comprising: a pulse transmissionelectromagnetic radiation source for illuminating a target area; aplurality of photosensors each responsive to radiation received from adifferent viewed area within the target area, each photosensor producingan electrical output signal which is a function of the instantaneousintensity of the radiation striking it; data reception means comprisinga separate channel connected to the output of each photosensor; eachchannel comprising analog to digital converter means for sampling thephotosensor output signal and producing a digital representation of theoutput signal amplitude; each channel being connected to memory meansfor storing the digital values and; data reduction means connected tothe memory means for receiving the digital values, said data reductionmeans including correlation means for obtaining the phase and peakcorrelation values between the responses of the different photosensors.